The present invention relates generally to a scanning electron microscope incorporating a field emission type electron gun and more particularly to an electron microscope which is capable of protecting an aperture plate from contamination due to collision of an electron beam therewith and which can always assure a high resolving power or resolution. Further, the present invention is also concerned with a method of controlling the aperture angle of a beam in a field emission scanning electron microscope (hereinafter also referred to simply as FESEM in abbreviation).
In a scanning electron microscope, an aperture plate for determining the aperture angle of the electron beam (also referred to as the beam aperture angle) at a specimen is disposed on or in the vicinity of the principal plane of a condenser lens. This aperture plate is susceptible to contamination due to collision of the electron beam therewith during the use or operation of the electron microscope. Such contamination can be explained by the fact that the degree of vacuum prevailing at the location where the aperture plate is installed, i.e. the vacuum prevailing between the condenser lens and the objective lens, is ordinarily on the order of 10.sup.-4 Pa (pascal) and thus particles floating in the ambience of the aperture plate are caused to deposit on the aperture plate through illumination of the electron beam. As such contamination progresses, astigmatism becomes more significant, being accompanied by a deviation of the optical axis, as a result of which the resolving power or resolution is considerably degraded. Accordingly, when the contamination becomes aggravated, the aperture plate has to be removed from the column of the electron microscope for cleaning. As an alternative measure, an electric heating device may be previously installed in association with the aperture plate for heating it in the state placed under vacuum within the column by electrically energizing the heating device to thereby eliminate the contamination by sublimation. However, a great difficulty is encountered in installing the aperture plate equipped with such heating device in a narrow space in the vicinity of the condenser lens. Thus, the second mentioned method is not adopted in general.
As an approach for solving the problems mentioned above, there has already been proposed such an apparatus in which an electron gun exchangeable by means of a specimen holder is provided on a specimen supporting stage within a specimen changer for thereby allowing the aperture plate to be cleaned through illumination by the electron beam emitted from the abovementioned electron gun, as is disclosed in Japanese Utility Model Publication No. 29739/1976.
As will be seen, every one of the prior art methods of cleaning the aperture plate is concerned with a post-processing after the aperture plate is contaminated and suffers from the problem that a lot of time is required for the cleaning.